Bixio Rimoldi

To code, or not to code: lossy source-channel communication revisited

What makes a source-channel communication system optimal? It is shown that in order to achieve an optimal cost-distortion tradeoff, the source and the channel have to be matched in a probabilistic sense. The match (or lack of it) involves the source distribution, the distortion measure, the channel conditional distribution, and the channel input cost function. Closed-form necessary and sufficient expressions relating the above entities are given. This generalizes both the separation-based approach as well as the two well-known examples of optimal uncoded communication. The condition of probabilistic matching is extended to certain nonergodic and multiuser scenarios. This leads to a result on optimal single-source broadcast communication.

To code or not to code

The theory and practice of digital communication during the past 50 years has been strongly influenced by Shannons separation theorem. While it is conceptually and practically appealing to separate source from channel coding, either step requires infinite delay in general for optimal performance. On the other extreme is uncoded transmission, which has no delay but is suboptimal in general. In this paper, necessary and sufficient conditions for the optimality of uncoded transmission are shown. These conditions allow the construction of arbitrary examples of optimal uncoded transmission (beyond the well-known Gaussian example).

Rate-splitting multiple access for discrete memoryless channels

It is shown that the encoding/decoding problem for any asynchronous M-user discrete memoryless multiple-access channel can be reduced to corresponding problems for at most 2M-1 single-user discrete memoryless channels. This result, which extends a similar result for Gaussian channels, reduces the seemingly hard task of finding good multiple-access codes to the much better understood task of finding good codes for single-user channels. As a by-product, some interesting properties of the capacity region of M-user asynchronous discrete memoryless channels are derived.

Design of coded CPFSK modulation systems for bandwidth and energy efficiency

Consideration is given to the problems related to the design of M-ary continuous-phase frequency-shift keying (CPFSK) systems with modulation index h=J/M, combined with eternal rate r binary convolution encoders. The following questions are raised and answered: (1) how should different encoder-modulator systems be compared and how can comparable systems be recognized from the system parameters, i.e. M, h, and r?; (2) what are the limits on the information rate per unit bandwidth, versus signal-to-noise ratio, when reliable transmission is required?; (3) how does one choose the system parameters M, h, and r when the overall system has to achieve a specified performance?; and (4) how does one design the external rate r binary convolutional encoder to put in front of the M-ary CPFSK modulation system with h=J/M? A simple approximation for the bandwidth of a CPFSK signal is given and shown to be sufficiently accurate for system design purposes. The design of the external convolutional encoder is carried out in a novel way that leads to fewer states in the combined encoder-modulator system and thus yields improved performance for a given demodulation-decoding complexity compared to previous approaches for the design of coded CPFSK systems. >

Approaching the AWGN channel capacity without active shaping

It is shown that the capacity of the AWGN channel can be approached via a multilevel coding scheme with the output of each encoder mapped into an independent signal constellation. No active shaping is required in this scheme, regardless of the signal-to-noise ratio. Moreover, shifted linear codes can be used.

Dense multiple antenna systems

We consider multiple antenna systems in which a large number of antennas occupy a given physical volume. In this regime the assumptions of the standard models of multiple antennas systems become questionable. We show that for such spatially dense multiple antenna systems one should expect the behavior of the capacity to be qualitatively different than what the standard multiple antenna models predict.

A Simple Converse of Burnashev's Reliability Function

In a remarkable paper published in 1976, Burnashev determined the reliability function of variable-length block codes over discrete memoryless channels (DMCs) with feedback. Subsequently, an alternative achievability proof was obtained by Yamamoto and Itoh via a particularly simple and instructive scheme. Their idea is to alternate between a communication and a confirmation phase until the receiver detects the codeword used by the sender to acknowledge that the message is correct. We provide a converse that parallels the Yamamoto-Itoh achievability construction. Besides being simpler than the original, the proposed converse suggests that a communication and a confirmation phase are implicit in any scheme for which the probability of error decreases with the largest possible exponent. The proposed converse also makes it intuitively clear why the terms that appear in Burnashevs exponent are necessary.

Dynamic Routing for Flying Ad Hoc Networks

This paper reports experimental results on self-organizing wireless networks carried by small flying robots. Flying ad hoc networks (FANETs) composed of small unmanned aerial vehicles (UAVs) are flexible, inexpensive, and fast to deploy. This makes them a very attractive technology for many civilian and military applications. Due to the high mobility of the nodes, maintaining a communication link between the UAVs is a challenging task. The topology of these networks is more dynamic than that of typical mobile ad hoc networks (MANETs) and of typical vehicle ad hoc networks. As a consequence, the existing routing protocols designed for MANETs partly fail in tracking network topology changes. In this paper, we compare two different routing algorithms for ad hoc networks: optimized link-state routing (OLSR) and predictive OLSR (P-OLSR). The latter is an OLSR extension that we designed for FANETs; it takes advantage of the Global Positioning System (GPS) information available on board. To the best of our knowledge, P-OLSR is currently the only FANET-specific routing technique that has an available Linux implementation. We present results obtained by both media-access-control (MAC) layer emulations and real-world experiments. In the experiments, we used a testbed composed of two autonomous fixed-wing UAVs and a node on the ground. Our experiments evaluate the link performance and the communication range, as well as the routing performance. Our emulation and experimental results show that P-OLSR significantly outperforms OLSR in routing in the presence of frequent network topology changes.

Generalized time sharing: a low-complexity capacity-achieving multiple-access technique

It is shown that the encoding/decoding problem for any asynchronous M-user memoryless multiple-access channel (MAC) can be reduced to corresponding problems for at most 2M-1 single-user memoryless channels. This is done via a method called generalized time sharing that is closely related to a previously developed method called rate-splitting multiple access. These methods reduce the seemingly hard task of finding good multiple-access codes and implementable decoders for such codes to the much better understood task of finding codes and decoders for single-user channels. As a by-product, some interesting properties of the capacity region of M-user asynchronous discrete memoryless channels are derived.

Varying the antenna locations to optimize the capacity of multi-antenna Gaussian channels

We investigate the problem of maximizing the capacity of multi-antenna Gaussian channels when one has the freedom of moving the transmit/receive antennas so as to modify individual singular values of the transfer matrix (subject to an overall energy conservation constraint). We solve this maximization problem and make some interesting observations. Namely that with n transmit and n receive antennas, when the signal-to-noise ratio is sufficiently large, the optimal solution corresponds to creating n parallel channels, all of which have the same strength. On the other extreme, when the signal-to-noise ratio is sufficiently small, the optimal solution boils down to creating a single channel. This is done by using the transmit and receive antennas to create focused beams. Perhaps the most interesting lesson is that beamforming is optimal only in the low signal-to-noise regime.